Conversion of ozone into hydroxyl radical by granular activated carbon with and without biofilms: Implications for...

The transformation of ozone (O₃) into hydroxyl radical (^•OH) during the ozonation was evaluated in the presence of granular activated carbon (GAC) and biofilm-covered granular activated carbon (BGAC). While both GAC and BGAC accelerated O₃ decomposition, the manner in which they generated ^•OH was different. GAC catalyzed the conversion of O₃ into ^•OH, with increasing hydroxyl radical exposure (∫[^•OH]dt) by 24 -72 % depending on the GAC dose. Conversely, BGAC exhibited limited capacity for ^•OH generation from O₃ decomposition, resulting in decreased ∫[^•OH]dt by 14 -25 % at higher BGAC doses. This disparity is likely attributable to biofilms on BGAC surface obstructing catalytic sites and hinder O₃-to-^•OH conversion. These different behaviors influenced the degradation of contaminants during ozonation. Specifically, GAC enhanced the degradation of O₃-resistant contaminants, whereas BGAC inhibited it. Machine learning (ML) models were developed based on experimental data to predict oxidant exposures (R² > 0.99 for ∫[O₃]dt and ∫[^•OH]dt). The elimination of several contaminants in systems was successfully predicted by these ML models, coupled with a straightforward kinetic equation that included adsorption and oxidation parameters. Additionally, ozonation modified the catalytic properties of GAC and BGAC. Extended ozonation oxidized GAC surface, diminishing its capability to convert O₃ into ^•OH. In contrast, oxidation of BGAC disrupted surface biofilms, thereby restoring its catalytic function.